The present invention concerns a doctor blade system for printing units as described in the preamble of claim 1. Also, the invention concerns a clamping device for a doctor blade.
In the present application, the invention will be explained specifically in connection with doctor blade chambers but may also be utilised in other doctor blade systems where a doctor blade is secured between a beam and a doctor rail.
In rotating printing units for offset printing, flexo printing etc. there is advantageously used systems including so-called doctor blade chambers. A doctor blade chamber is an ink container which by means of so-called doctor blades fits tightly to an ink transfer roller, and from which container ink is transferred to this roller, often a raster roller transferring the ink to further rollers forming a part of the printing process.
The doctor blade chamber, which may be several meters long, consists mainly of a doctor beam constituting the wall of an ink chamber, in the front side of which, facing the roller, there is an open channel and two or more doctor blades clamped onto the doctor beam. These doctor blades are thin, elongate blades having one longitudinal side firmly connected to the doctor beam and with their opposite sides resting under a certain spring force against the roller.
Often two doctor blades are used in the doctor blade chamber system, where one doctor blade serves for sealing for the ink chamber against the roller, and the other serves for sealing the ink chamber against the roller as well as supplying the roller with an even layer of ink; these functions are advantageous as compared with system where the roller collects ink from an open ink container as in that case it is difficult to achieve an even and precise ink transfer. Besides, an open ink container has the disadvantage that a part of the ink is sprayed around the roller at the collection due to the great speed of the roller, thereby contaminating the printing unit.
The doctor blades, which, in spite of this common name, may also be made of synthetic material, are wear parts in a printing unit. A doctor blade is a thin blade resting on the ink transfer roller, for example the raster roller.
Depending on the quality of the roller and on the ink, a doctor blade lasts between one day and several weeks after which it has to be replaced. For replacing the doctor blade, according, to prior art the doctor blade chamber usually has to be taken out of the printing unit which is a disadvantage. After demounting a clamping rail disposed on the beam of the doctor blade chamber and keeping the doctor blade clamped between the rail and the bean, the doctor blade may be replaced after which the clamping rail may be mounted again.
According to prior art, this clamping rail is screwed onto the doctor blade chamber beam with a number of screws, implying some disadvantages. First, quite a number of screws (10 screws per meter of doctor blade chamber) have to be loosened and tightened, making the replacing process slow. Second, it is a well-known problem that the clamping rail does not clamp quite evenly on the doctor blade as the screw system implies a raised pressure right around the screws. The consequence is that the doctor blade does not lie evenly to the roller but xe2x80x9cflickersxe2x80x9d, i.e. has shape like a wave. These variations in distance between the doctor blade and the roller, even though they are very smallxe2x80x94in the magnitude of few micronxe2x80x94, result in the application of ink on the roller not being quite even, thereby depreciating the printing quality. Furthermore, at the areas between the screws ink may penetrate between the doctor blade and the clamping rail and between the doctor blade and the doctor beam, further enhancing the xe2x80x9cflickeringxe2x80x9d effect and requiring frequent cleaning. Third, tools are required for loosening and tightening the screws. During work it may be a great source of irritation for the workers if this tool is not available for the replacement, for example because a colleague has mislaid it.
A quick replacement of the doctor blade is achieved with a mechanism as described in American patent U.S. Pat. No. 5,517,918 where the doctor blade is secured by means of a couple of leaf springs. The leaf springs overlap some of the doctor blade and is prestressed so that the doctor blade is pressed against the beam. By replacing the doctor blade there is used a displacing mechanism in the form of a resilient tube, which may be expanded automatically whereby the springs are pushed away from the doctor blade which is thereby loosened.
This mechanism has the disadvantage that the springs are fastened by a row of screws in analogy to the holding of the clamping rail in traditional doctor blade chambers. As the springs are a lot thinner that traditional clamping rails, the above flicker-effect is still more pronounced than in traditional printing units, making this system unsuitable for print where high quality is demanded.
Another system has been described in American patent U.S. Pat. No. 3,085,275 where the doctor blade is secured between the clamping rail and the beam by the force from a pneumatic expanding tube is transferred to the clamping rail by means of a tip mechanism. In this system it is not the holding mechanism itself which causes flickering in the doctor blade but a guide rail provided on the doctor blade, the guide rail being fastened with screws to the doctor beam. Furthermore, in this system it is disadvantageous that the doctor blade is fastened to a rail which together with the doctor blade has to be pushed laterally out of the chamber when changing doctor blade. As printing units may be very long, much space beside the printing unit is required. A further drawback is that the printing unit only functions satisfactorily if there is sufficient pneumatic pressure. By pressure failure the printing unit will not be able to work satisfactorily any more.
Different pressure systems exist where the doctor blade is clamped between the clamping rail and the beam without flicker appearing. Such a system is described in American patent publication U.S. Pat. No. 4,938,131. By pneumatic expansion of a flexible tube, the clamping rail is pressed against the doctor beam whereby the doctor blade is secured therebetween. This system has, however, the great disadvantage that the doctor blade is no longer secured if failure of the pneumatic supply occurs. This implies a great risk of danger as the doctor blade, due to the rotating printing roller, may be flung out of the doctor with great speed with risk of damage to equipment and people around the printing unit.
The purpose of the invention is to provide a doctor blade chamber with a clamping device for doctor blades which does not have the above drawbacks.
This purpose is achieved with the present invention by a doctor blade system of the kind mentioned in the introduction, and which is peculiar as described in the characterising part of claim 1.
Such a clamping device may be used in doctor chambers with a doctor blade, e.g. single doctor blades for use in intaglio or screen process printing, or in doctor devices with two or more doctor blades, e.g. doctor blade chambers for rotary printing.
By designing the beam of the doctor blade chamber and the clamping rail with a groove, preferably T-shaped, in which there is mounted a suitable, for example T-shaped, sectional rail, there is achieved a continuous connection between the clamping rail and the beam not having the drawbacks of the previously described pointed connection where screws are used.
To achieve clamping of the doctor blade, the beam is designed with a groove in which the sectional rail may be displaced in direction against the bottom of this groove so that by this displacement a clamping of the doctor blade between the clamping rail and the doctor blade chamber is effected. The trouble implied in loosening and tightening the row of screws is avoided by this device.
In practice, the displacement of the sectional rail is effected in the way that, for example, there is inserted one or more wave springs between the inner wall of the beam groove, i.e. the wall opposite to the bottom of the groove, and the sectional rail so that the spring or springs press the sectional rail in direction against the bottom of the beam groove. Thereby the doctor blade is clamped between the clamping rail and the beam.
The wave springs may also be substituted by elastic polymers, e.g. designed as a tube (rubber tube) or a foam polymer. It is crucial that eventually a force action is achieved so that it is clamped between the clamping rail and the beam.
In order to loosen the doctor blade again from its clamped position, the sectional rail in the beam is to be pushed in direction away from the bottom of the beam groove. This is achieved with a displacing mechanism in the doctor blade chamber which by actuation counteracts the force of the springs and thereby presses the sectional rail in the beam groove away from the bottom of the beam groove.
In practice, this is achieved by e.g. filling the space between the sectional rail and the bottom of the beam groove with gas under high pressure, e.g. pressurised air. In order that overpressure may be maintained in this space, the ends of the beam groove is sealed with specially designed packings.
Another possibility is to dispose a resilient tube, e.g. a rubber tube, in the interspace between the sectional rail and the bottom of the beam groove which may be expanded pneumatic or hydraulic in order thereby to counteract the force of the springs.
A further possibility, which is mechanical, is to place a rigid, oval tube being rotatable about its longitudinal axis in the interspace between the sectional rail and the bottom of the groove. By turning the tube the distance between the sectional rail and the bottom of the beam groove is increased.
A suitable feature in the invention is that the clamping of the doctor blade is passive and that the displacing mechanism is active when the doctor blade is loosened from its clamped position in the sense that an action has to be performed, e.g. filling of the interspace with pressurised air or using pneumatics or hydraulics for loosening the doctor blade. This principle has been chosen out of safety considerations as this is advantageous as compared with a device, e.g. hydraulic, where the doctor blade is secured by an active process. By an active clamping device there will be the possibility of the doctor blade suddenly not being clamped any more by pressure failure in the pneumatics or hydraulics, whereby there is the possibility that the razor-sharp doctor blades are flung out of the printing unit and across the print shop, implying danger for the persons around the printing unit.
In a further embodiment of the sectional rail, the side engaging the clamping rail has an edge designed with cross-section as a hook; thereby the clamping rail may be lifted free of the sectional rail when the sectional rail is pressed out of the groove by the pneumatic, hydraulic or mechanical displacing mechanism. This design is advantageous when the interspace between the clamping rail and the doctor beam is to be cleaned as time-consuming demounting and mounting is avoided.
In a further embodiment of the invention, the sectional rail is resilient and designed so that the elastic force itself in the sectional rail draws the clamping rail toward the beam whereby the doctor blade is clamped. For counteracting the effect of the elastic sectional rail when doctor blade is to be exchanged, the above displacing mechanism is pneumatic or hydraulic. As mechanical alternative in this case it is, however, possible to use an oval, rigid tube being rotatable about its longitudinal axis.
In a further development, the elastic sectional rail is designed in two parts that are pushed into the beam groove from each their end of the beam groove. The lengths of the two parts are chosen in such a way that their ends are in contact with each other in the groove of the doctor beam. This design has the advantage of these two parts furthermore each being able to be designed with a sealing at one end so that the beam groove is sealed at the ends when the elastic sectional rail is provided therein. In this way it is ensured that ink and dirt are prevented from entering at the ends in the beam groove and that the overpressure may be maintained in the interspace between the sectional rail and the bottom of the beam groove when the above mentioned displacing mechanism in the design where the interspace is filled with gas under high pressure is used for pressing the sectional rail away from the bottom of the beams groove when the doctor blade is to be demounted.
In a further embodiment of the invention, the doctor beam is designed with an inner chamber extending along the beam in the part of the beam on which the clamping rail is mounted for clamping the doctor blade. This inner chamber has a resilient wall being the one bearing against the clamping rail. The elastic force in this wall draws the wall inward toward the inner chamber. The elastic wall has a sectional bead at its outer side, e.g. T-shaped or dovetailed, for engaging a correspondingly designed groove in the clamping rail. Additionally, the clamping device comprises a displacing mechanism which by actuation presses this resilient wall outward so that the clamping rail may be mounted with the sectional edge of the elastic wall engaging the groove of the clamping rail. By deactuation of this displacing mechanism, the wall is again drawn inward against the inner chamber whereby the clamping rail is drawn toward the beam and the doctor blade is clamped between the clamping rail and the beam.
This displacing mechanism may be provided by filling the inner chamber with gas or liquid under high pressure for counteracting the elastic, inward directed force. Another possibility is to provide an elastic tube in the chamber, which tube may be expanded pneumatic or hydraulic. A third possibility is to provide a rigid, oval tube being rotatable about its longitudinal axis in the chamber which by rotation presses the wall outward so that the clamping rail is loosened from its clamping.
The making of the doctor beam with an inner chamber may, for example, occur by extruding the beam in metal or synthetic material.
Alternatively, the inner chamber may be provided by making the beam with a groove which is then covered with an elastic rail having an elastic force inward toward the beam groove. At its outer side, the rail is equipped with a sectional bead, for example T-shaped or dovetailed, for engaging a corresponding groove in the clamping rail.
In a further embodiment of the invention, the clamping rail is designed with an edge along the clamping rail. The clamping rail is placed on the beam so that the edge of the rail engages a groove on the beam suitably shaped thereto, and so that the clamping rail may tip about this edge.
Between a first part of the clamping rail and the beam there is provided an elastic, oval tube which by its elastic force presses this first part away from the beam whereby the clamping rail tips about the edge and the second part of the clamping rail presses against the beam whereby the doctor blade is clamped between this second part and the beam. For loosening the doctor blade from the clamped position, the elastic, oval tube may be filled with gas or liquid which subjected to high pressure reduces the ovality of the tube, i.e. it becomes more round, resulting in the distance between the first part of the clamping rail and the beam is reduced whereby the doctor blade is no longer clamped. The elastic tube may now easily be removed from the doctor blade chamber whereafter also the clamping rail may be removed without requiring further demounting. Therefore, this design is very suitable when the interspace between the clamping rail and the beam is to be cleaned.
As substitution for the use of a pneumatic or hydraulic deformable tube between the first part of the clamping rail and the beam, a rigid, oval tube may be used. In this case, the distance between the first part of the clamping rail and the beam may be changed by turning the tube about its longitudinal axis. Suitably, at handle is used thereto. In order that this handle is not by actuated by accident, resulting in loosening of the doctor blade, the tube is turned more than its upper dead point, i.e. the position where there is maximum distance between the clamping rail and the beam. Even by very little sliding resistance, for example because of ink between the tube and the clamping rail or the beam, respectively, the tube will not unsuitably turn by itself, thereby loosening the doctor blade, as a force has to be exerted actively for turning the tube over the dead point for loosening the doctor blade.
In a further design, the handle is disposed displacing on the tube, and the doctor blade chamber beam has been provided with a hole in which the handle may be provided when it is not in use. In this way it is prevented that the handle is actuated by mistake so the doctor blade is loosened. In order that the handle does not unsuitably slide out of the hole, the hole is e.g. provided with a ball lock. This ensures that the doctor blade is retained until an action is performed for loosening the doctor blade.
The tube with handle may advantageously be designed so that in two opposite positions it causes loosening of the doctor blade but only in one position it is possible to push the tube out of the doctor blade chamber. In this way it is ensured that the tube with the handles in position for loosening the doctor blade does not unsuitably slide out of the doctor blade chamber.
In a further embodiment, the handle on the rigid, oval tube is disposed pivotable as well as sliding on the tube so that it may be pushed into the tube itself when the handle is not in use. In order that the handle does not unsuitably slide out of this hole, the hole is, for example, provided with a ball lock.
For practical reasons the tube may be designed in two parts that are pushed into the doctor blade chamber from each their end.
A design using a tube with handles will preferably find application for doctor blade chambers of lesser width, typically 1-1.5 m, while for wider doctor blade chambers it is an advantage to use the solution including pneumatics or hydraulics, possibly in connection with resilient tubes.
A further advantage by the said designs is that the doctor blade chamber is not to be demounted from the printing unit when the doctor blade is to be replaced. Thereby the replacing procedure becomes easier and quicker.
The invention is explained more closely in the following with reference to the drawing, where